1. Field of the Invention
The present invention relates to anaerobically curable compositions that provide enhanced properties when cured on an inactive surface. The compositions cure on inactive surfaces without the use of a primer and without the addition of a transitional metal catalyst.
2. Brief Description of the Related Technology
Anaerobic adhesive compositions are well known for their ability to remain in a liquid, unpolymerized state in the presence of oxygen and to cure to a solid state upon the exclusion of oxygen. Early work on anaerobic adhesive compositions concentrated on developing a cure system which improved the speed and/or bond strength of the adhesive composition. Various cure systems for anaerobic adhesive compositions have been developed to primarily focus on efficiently performing the redox reaction, which is the basis for anaerobic chemistry. Hydroperoxides were found to serve as a catalyst for the generation of a free radical.
Anaerobic compositions generally contain (meth)acrylic functional monomers, an organic hydroperoxy or perester initiator, accelerators such as saccharin and/or dimethyl-p-toluidene, and stabilizers such as hydroquinone or other phenolic stabilizers and metal chelators such as sodium EDTA. Those persons of ordinary skill in the art acknowledge that peroxides serve as a free radical generating source which initiate free radical curing of the polymerizable anaerobic adhesive monomer compositions. To increase the speed at which the free radical is generated, accelerators are employed in combination with the peroxides to enhance the speed at which the peroxide free radical is generated. In so doing, the cure speed of the anaerobic adhesive composition is increased. Speed of cure and strength are two desirable properties of anaerobic sealants that increase their usefulness for impregnation of porous materials and threadlocking.
Substrates such as stainless steel, zinc, dichromate, cadmium and plastic are inactive porous materials that are relatively slow to cure compared to active metals such as iron and copper. Therefore, many anaerobic adhesive compositions have conventionally employed primer compositions to speed their cure and/or the addition of a transition metal, generally in the form of a salt. For inactive surfaces, primer compositions have been considered necessary for the quick fixture and cure times required of many applications.
The use of primer compositions requires an additional step prior to applying the anaerobic adhesive composition, which is often inconvenient and costly. Often the solvent used to carry the accelerator component in the primer is environmentally harmful and requires special handling and disposal. Moreover, the user must ordinarily wait until the solvent has evaporated before applying the adhesive. For example, U.S. Pat. No. 4,731,146 to Clark discloses a primer/activator composition that requires the use of a volatile organic solvent to carry the cure activator which must evaporate before the anaerobically curable composition may be introduced to the substrate.
Limiting the number of components required to provide an anaerobically curable composition promotes efficiency of manufacturing and reduction of cost. A further advantage would be to provide an anaerobically curable composition with enhanced properties such as speed of cure and strength of cure without the need for a primer or the addition of a transitional metal. Therefore, there is a need for an anaerobically curable composition that provides enhanced cure properties with a limited number of components required in manufacture that does not require the additional step of preparing the substrate with a primer and may be used without the addition of a transitional metal.